Issue 18

G. Ferro et alii, Frattura ed Integrità Strutturale, 18 (2011) 34-44 ; DOI: 10.3221/IGF-ESIS.18.04 41 The interface adhesion can be improved by chemically functionalizing the nanotubes surfaces; this generates strong covalent-type of bonding [13]. Thus, the bonding between nanotubes and cement paste allows the strain to be transferred from one stage to another and crack bridging becomes effective. It is known in the literature that carbon nanotubes treated by using a H 2 SO 4 and HNO 3 mixture solution (3:1 by volume, respectively) at room temperature and under ultrasonication may lead to the formation of carboxyl acid groups on their surfaces (COOH-MWCNTs). Li et al. [27] reported that carboxylic groups are effective in increasing the compressive strength up to 19%, while the flexural strength increases up to 25%, after addition of 0.5 wt% of COOH-MWCNTs [13]. Due to the presence of carboxylic acid groups on the surfaces of carbon nanotubes when modified by using the mixed solution of sulfuric acid and nitric acid, chemical reactions take place between the carboxylic acid and the calcium silicate hydrate (C–S–H) or Ca(OH) 2 (Fig. 11). However, it seems that the number of carboxylic groups attached to the surface of carbon nanotubes is crucial. Musso et al. have evidenced that the flexural strength of prisms (40×40×160 mm 3 ) was divided by a factor of 2.5 with respect to the plain cement (Fig. 12), after addition of 0.5 wt% (with respect to the cement) of MWCNTs with 4 wt% of -COOH groups [17]. As described into the literature [28], oxidative acid treatment can strongly increase the CNTs wettability. As a consequence, depending also on the diameter of the hollow cavity, functionalized CNTs can strongly suck inside aqueous solutions by means of capillarity behavior favored by hydrophilic character [29]. As confirmed by the analysis performed on COOH-MWCNTs used to produce concrete-composite, the high amount of lattice defects and carboxylic groups can justify a strong hydrophilic behavior (Fig. 13) that is probably responsible for the incomplete hydration of cement paste added with nanotubes which initially retained the water during concrete preparation and then released it progressively during air curing. Figure 11 : Reaction scheme between carboxylated nanotube and hydrated phases of cement (Ca(OH) 2 and C–S–H) [13]. Figure 12 : Flexural strength of mortars as such and with 0.5 wt% of COOH-MWCNTs [17] 1 2 3 0 2 4 6 8 10 12 Flexural strength (MPa) Sample Reference mortar Mortar w COOH-MWCNTs